1. Field of the Invention
The present invention relates to an optical pickup apparatus for reading information from a plurality of optical discs to be respectively read in different reading wavelengths, and more particularly, to an optical pickup apparatus which includes a semiconductor light source for emitting a plurality of laser beams of different wavelengths and a photodetector for detecting the plurality of laser beams.
2. Description of the Related Art
There have been various proposals relating a compatible disc player for reproducing a CD (Compact Disc) and a DVD (Digital Versatile Disc) in which an optical pickup is commonly used to reproduce the CD and the DVD. The DVD/CD compatible disc player employs an optical pickup having, for example, a single-wavelength bifocal lens or a bifocal lens.
In comparison of the CD with the DVD in terms of a disc structure, the DVD disc has a protection layer, the thickness of which (for example, 0.6 mm) is approximately one half the thickness of the protection layer of the CD disc. Thus, the following problems arise when both discs are reproduced by a common optical pickup employing a monofocal lens. Aberration such as spherical aberration occurs in the light beam due to the protection layer of the CD disc which is thicker than that of the DVD disc, through which the light beam passes, when a light beam is converged to be optimal for an information recording surface of the DVD disc. Therefore, the light beam cannot be optimally converged for an information recording surface of the CD disc.
In addition, it is necessary to form a beam spot of a size optimized for the size of the information pits on the information recording surface in order to correctly read the respective information pits since the CD disc differs from the DVD disc in the size of information pits formed for recording.
The size of the beam spot is proportional to the ratio of the wavelength of a laser beam to the numerical aperture of an objective lens for converging the laser beam on the information recording surface. With a fixed wavelength of a laser beam, the beam spot is smaller as the numerical aperture is larger. Therefore, the beam spot excessively smaller for information pits, for example, on the CD disc when the CD and the DVD are reproduced by means of a common optical pickup employing a monofocal lens and the numerical aperture is adapted to information pits on the DVD disc with a fixed wavelength of the laser beam. Thus, a reproduced signal suffers distortion when the CD is reproduced, thereby making it difficult to correctly read the information pits on the CD.
To overcome the above-described problem, the DVD/CD compatible reproducing apparatuses mainly adopt an optical pickup employing a bifocal lens which is capable of focusing at different positions on the same straight line and irradiating two laser beams for forming beam spots of sizes appropriate for the sizes of the respective information pits.
For example, an optical pickup apparatus illustrated in FIG. 1 combines light beams from a first light source 10 for a CD and a second light source 15 for a DVD with a first beam splitter 13 which is a combining prism, and has a bifocal lens comprised of an objective lens and a diffraction element. A DVD/CD compatible reproducing apparatus having the optical pickup apparatus will be described below in terms of configuration and operation.
In FIG. 1, a first light source 10 generates a laser beam (indicated by broken lines) of a wavelength (780 nm) optimal for reading information from the CD in response to a driving signal from a first driver circuit 11. The laser beam is irradiated to the first beam splitter 13 through an optical grating 12 for generating three beams. The first beam splitter 13 reflects the laser beam from the first light source 10 to direct the reflected light to a second beam splitter 14.
The second light source 15 positioned at 90 degrees with respect to the first light source 10, on the other hand, generates a laser beam (indicated by solid lines) of a wavelength (650 nm) optimal for reading information from a DVD in response to a driving signal from a second driver circuit 16, and irradiates the laser beam to the first beam splitter 13 through an optical grating 17. The first beam splitter 13 transmits the laser beam from the second light source 15 to direct the same to the second beam splitter 14.
The second beam splitter 14 directs the laser beam supplied through the first beam splitter 13, i.e., the laser beam from the first light source 10 or the second light source 15 to a bifocal lens 19 through a collimator lens 18. The bifocal lens 19 converges the laser beam from the second beam splitter 14 to a single point, and irradiates the converged laser beam, as information reading light, to an information reading surface of a disc 21 which is driven by a spindle motor 20 to rotate.
The laser beam (indicated by broken lines) from the first light source 10 is converged by the bifocal lens 19 so as to focus on an information recording surface C of the disc 21. The laser beam (indicated by solid lines) from the second light source 15 in turn is converged by the bifocal lens 19 so as to focus on an information recording surface D of the disc 21.
Reflected light produced by irradiating the information reading light from the bifocal lens 19 to the disc 21 passes through the bifocal lens 19 and the collimator lens 18, is reflected by the second beam splitter 14, passes through a cylindrical lens 22 serving as an astigmatism generating element, and impinges on a photodetector 23. The photodetector 23 generates an analog electric signal corresponding to the amount of incident light, and supplies the analog electric signal to an information data reproducing circuit 24 and a disc discriminating circuit 25 as a read signal.
The information data reproducing circuit 24 generates a digital signal based on the supplied read signal, and applies demodulation and error correction to the digital signal for reproducing information data. The disc discriminating circuit 25 identifies the type of the disc 21 based on the size of a beam spot which is formed when the disc 21 is irradiated with the laser beam, for example, as disclosed by the same applicant in Laid-open Japanese Patent Application Kokai No. H10-255274, and supplies the identified type to a controller 26. The controller 26 controls in response to a disc identifying signal to selectively drive either the first driver circuit 11 or the second driver circuit 16. The controller 26 only drives the first driver circuit 11 when it is supplied with a disc identifying signal of CD from the disc discriminating circuit 25. Therefore, the laser beam emitted from the first light source 10 is irradiated to the disc 21 through the optical system including the grating 12, the first beam splitter 13, the second beam splitter 14, the collimator lens 18 and the bifocal lens 19.
On the other hand, the controller 26 only drives the second driver circuit 16 when it is supplied with the disc identifying signal of DVD from the disc discriminating circuit 25. Therefore, the laser beam emitted from the second light source 15 is irradiated to the disc 21 through the optical system including the grating 17, the first beam splitter 13, the second beam splitter 14, the collimator lens 18 and the bifocal lens 19.
Stated another way, the optical pickup apparatus is configured to include the first light source 10 for generating a laser beam having a wavelength optimal for reading information from the disc 21 such as a CD which has a relatively low recording density, and the second light source 15 for generating a laser beam having a wavelength optimal for reading information from the disc 21 such as a DVD which has a high recording density, so as to alternatively use the one corresponding to the type of the disc 21 from which information is reproduced. Reflected light (return light) reflected from the information recording surface of the disc 21 passes through the bifocal lens 19 and the collimator lens 18, is reflected by the second beam splitter 14, passes through the cylindrical lens 22, and impinges on the photodetector 23.
As described above, in the DVD/CD compatible reproducing apparatus having two light sources, when the first light source 10 is irradiated from one surface of the first beam splitter 13, the second light source 15 must be irradiated from another surface perpendicular to the first light source 10, causing a problem that a large space is required for accommodating the optical system and accordingly the optical pickup apparatus has a larger size.